Method of preparing dental ceramics for bonding

ABSTRACT

A method of preparing the fitting surface of a dental ceramic body for subsequent bonding to a tooth with, for example, glass polyalkenoate and resin based cements, including the step of depositing, by a vapour phase deposition technique, directly onto the fitting surface of the ceramic body a strongly adherent coating of an inorganic substance such as tin oxide at a thickness ideally less than 2 microns, the coating being reactive with the cement to provide a durable chemical bond therewith which is not subject to chemical degradation in the oral environment and which will not compromise the aesthetics of the associated dental restoration.

This invention concerns a method of preparing a surface of a dentalceramic body for subsequent fitting to a tooth, by means of improvingthe bond strength between the fitting surface of the body and certaincommercially available dental cements.

Ceramics including high-alumina porcelains and glass ceramics arefrequently used in restorative dentistry for the construction of, forexample, jacket crowns, inlays or aesthetic veneers. Such materials areparticularly useful for these purposes, being chemically inert andbio-compatible in the oral environment, and are aesthetically pleasing.

However, one disadvantage in the use of ceramics is that their inertchemical nature renders them unable chemically to bond directly tocertain dental cements such as glass polyalkenoate cements.Consequently, mechanical retention techniques are often used forattachment. Whilst mechanical bonding may be enhanced by etching thefitting surface of the ceramic material to provide a "key" this tends tocause stress concentration locally and potentially weakens the ratherbrittle material.

One known technique developed to strengthen porcelain jacket crowns, hasthe additional advantage of overcoming the problem of bonding. Itinvolves the preparation of a "thimble" of platinum foil, coated on bothsides with a layer of tin oxide. The ceramic is fired onto the oxidecoated outer surface of the thimble whilst the oxide coated innersurface provides a chemically reactive surface for bonding to thecement. Whilst this technique is effective, it involves a multi-stageprocess and the use of expensive platinum foil, and compromises theaesthetics of the restoration.

Other chemical methods of bonding to dental porcelain exist. The firstis a group of chemicals called silane coupling agents, which can beinitially effective in bonding to resin-based cements, but are subjectto chemical degradation in the oral environment.

An object of the present invention is to provide a method of preparingthe fitting surface of a dental ceramic body for subsequent bonding to atooth using a dental cement, in which the aforementioned disadvantagesare avoided. Such a method, according to the present invention,comprises the step of depositing by a vapour phase deposition technique,directly onto said fitting surface of the ceramic body, a stronglyadherent coating of an inorganic substance which is capable of reactingwith the cement to provide durable bonding to such cement.

Various chemically reactive species are available for this purpose,including, for example, metals and alloys; metal salts; metallic andnonmetallic compounds; glasses, particularly reactive or ion-leachableglasses; and other reactive ceramics. In a specific example, tin oxideis capable of being deposited as a microns-thin layer (ideally less thantwo microns) directly onto the fitting surface of the dental ceramic.

The selection of a coating material suitable for this purpose isdetermined such that it shall be bio-compatible, and, with reference toacceptable optical qualities, to be preferably white or off-white orcolourless so as not to compromise the aesthetics of the restoration.Since the marginal regions of a ceramic body prepared in accordance withthe invention will, in use, be subjected to saliva, acids of bacterialorigin and acids originating from both foods and regurgitated gastriccontents, it is important to avoid dissolution in the edge regions owingto chemical degradation, which in turn could lead to bond failure. Thesubstance must be insoluble in both cold and hot water and should resistdissolution in low pH environments.

Various methods exist for vapour phase deposition of such materials ontoceramic surfaces, including physical vapour deposition techniques suchas evaporation and sputtering, ion implantation and chemical vapourdeposition.

Of sputtering techniques perhaps the most appropriate is the ion platingtechnique which incorporates partial ionisation of the depositionspecies thus increasing their average energy and enabling betterintegration of the coating with the substrate. Ion plating (commonlyknown as "plasma assisted PVD") is a process in which ion bombardment ofthe substrate may be accomplished by making the substrate a cathode of alow pressure plasma discharge in a mixture of the coating flux and aninert working gas (usually Argon).

The coating may be applied in a two-stage process in which an ionplating technique is used initially to deposit a metal or alloy on thefitting surface of the ceramic body. Such metal/alloy may then bereacted with other elements or compounds or polyanions in order toproduce the desired coating. Examples of such post deposition processesinclude, (i) oxidation in an oxygen containing atmosphere at elevatedtemperatures, and (ii) additional bonding of the coating, for example bylaser processing.

In a direct coating method reactive ion plating may be used to deposit acoating directly onto the ceramic surface.

Two or more such substances may be deposited in a composite structure inorder to combine the advantageous features of such substances.

The chemical and physical properties of the coating may be modifiedafter deposition in order to improve still further its adhesioncharacteristics. For example, the chemical properties, the colour, orthe bond strength may be modified by heating a tin oxide layer to reduceor oxidise the coating.

Experimentally, tensile bond strength tests have been carried out onaluminous ceramics pre-coated in accordance with the invention, using aresin based cement. When such tensile testing was carried out on dentalporcelain samples, the mode of failure was predominantly cohesive withinthe porcelain. However, in the case of high alumina-containing ceramics,the primary mode of failure in the well oxidised tin layers was mainlycohesive within the cement.

Examples of such experimental strength tests will now be referred to. Ineach test the resin-cement used was a phosphate methacrylate cement.

Initially in a controlled experiment common dental porcelain was bondedto a substrate consisting of a nickel chromium rod, and after a periodof 7 days the specimen was tested in tension until failure. The strengthof the bond between the untreated porcelain and the cement was measuredat 3.4 MPa (megapascals). The sample failed due to poor adhesion at thebond interface with the porcelain.

In a second example a dental porcelain sample was pre-coated inaccordance with the invention by deposition of tin oxide onto thefitting surface using a reactive ion plating technique with an RF powersupply. Failure occurred at 7.8 MPa, and it is believed that failureoccurred cohesively within the porcelain. It can be seen that the bondstrength was therefore considerably higher as a result of deposition inaccordance with the invention.

In a third example a further controlled experiment was carried out thistime with a high alumina-containing ceramic body bonded to a nickelchromium rod. Here, a bond strength of 11.4 MPa was recorded. Adhesivefailure between the ceramic and the cement was seen to have occurred.

In a fourth example, a similar high alumina-containing ceramic body waspre-coated in accordance with the invention with a deposit of tin oxideusing a reactive ion plating technique with an RF power supply. In thiscase, a bond strength of 26.7 MPa was recorded, i.e. something like 21/2times greater than the third example in which no deposition waseffected.

In a fifth example a similar high alumina-containing ceramic body waspre-coated with a deposition of tin using a direct current sputteringtechnique and this yielded a bond strength of 12.0 MPa.

In a sixth example the substrate and deposition material were the sameas in the previous example except that in this case post-depositionoxidation of the tin coating was carried out thus yielding a bondstrength of 17.9 MPa.

In a seventh example a high alumina-containing ceramic body wassubjected to deposition of a tin coating, this time by a magnatronsputtering technique and a bond strength of 15.8 MPa was recorded.

In an eighth example in which the ceramic body and coating material werethe same as in example seven, post-deposition oxidation of the tincoating was carried out and was found to yield a bond strength of 17.9MPa.

It will be seen from the above examples that tensile bond strengthbetween a ceramic body and a dental cement is substantially improved bydepositing, by a vapour phase deposition technique, directly onto thefitting surface of the ceramic body, a strongly adherent coating of aninorganic substance which is capable of reacting with the cement toprovide a durable bond. It is also clear that there are severaleffective methods of applying the coating and that the technique will bechosen according to the materials selected for the substrate and for thecoating. Improvements can be made in certain cases by enhancement of thecoating by post-deposition oxidation.

The process of coating the fitting surface of a dental ceramic body mayalso serve to improve the mechanical integrity of the fitting surfacegenerally.

I claim:
 1. A method for preparing the fitting surface of a dentalceramic body for subsequent bonding to a tooth, the method comprisingthe steps of:depositing by a vapor phase deposition technique, directlyonto said fitting surface of the ceramic body, a strongly adherentcoating of a metal oxide; and applying to said metal oxide coating aphosphate methacrylate dental cement which reacts with said metal oxidecoating to provide durable bonding of the body to such cement.
 2. Amethod of bonding a dental ceramic body to a tooth, comprising preparingthe fitting surface of the body by a method according to claim 1, andfitting the prepared surface to the tooth.
 3. A method according toclaim 1, wherein said coating is deposited as a layer of less than 2microns in thickness directly onto said fitting surface of the ceramicbody.
 4. A method according to claim 1, wherein said metal oxide isselected to be insoluble in water and to resist dissolution in a low pHenvironment.
 5. A method according to claim 1, wherein said coating iseffected by physical vapour deposition.
 6. A method according to claim1, wherein said coating is effected by ion plating.
 7. A methodaccording to claim 1 wherein said coating is effected by chemical vapourdeposition.
 8. A method according to claim 1, wherein said coating iseffected by ion implantation.
 9. A method according to claim 1, whereinsaid coating is effected by direct current sputtering.
 10. A methodaccording to claim 1, wherein said coating is effected by magnetronsputtering.
 11. A method according to claim 1, wherein the coating afterdeposition on the ceramic body is further treated to enhance its colourand its bond strength.
 12. A method according to claim 4, wherein saidmetal oxide is tin oxide.